1. Field of the Invention
The present invention relates to a voltage regulator circuit and a control method therefor.
2. Description of the Related Art
Recently, portable equipment that uses a battery, such as a mobile telephone, has come into widespread use. Such portable equipment generally employs a voltage regulator to maintain a constant voltage level. To improve load response characteristics of the voltage regulator, a voltage regulator circuit that amplifies an AC (alternating current) component of an output voltage for feedback to an output transistor is proposed.
FIG. 1 is a diagram illustrating example circuitry of such a voltage regulator circuit. The voltage regulator circuit 100 of FIG. 1 converts an input voltage Vin applied to an input terminal IN into a constant voltage and outputs an output voltage Vout from an output terminal OUT. The voltage regulator circuit 100 includes a first error amplifier 101 and a second error amplifier 110.
The first error amplifier 101 amplifies a voltage difference between a reference voltage Vref and a divided voltage VFB generated by dividing the output voltage Vout by resistors R101 and R102, which is then output to the gate of an output transistor M101, thereby controlling a current output from the output transistor M101 to maintain the output voltage Vout constant.
The second error amplifier 110 is an amplifier that responds faster than the first error amplifier 101 and has an input terminal connected to the output terminal OUT and an output terminal connected to the gate of the output transistor M101. The second error amplifier 110 amplifies an AC component of the output voltage Vout and controls the gate voltage of the output transistor M101. That is, the second error amplifier 110 amplifies a change in the output voltage Vout caused by fluctuation in load current and responds to control the gate voltage of the output transistor M101 faster than the first error amplifier 101 does, thereby greatly improving transient response characteristics.
However, bias current of the second error amplifier 110 is determined to be larger to achieve faster operation than that of the first error amplifier 101, resulting in increased current consumption. In particular, when the voltage regulator circuit 100 is used as a power source for a system having a heavy-load operating mode with normal current consumption and a light-load operating mode such as a sleep mode with low current consumption, the voltage regulator circuit 100 needs to have quick transient response characteristics for changes in load condition even in the light-load operating mode. When current consumption of the second error amplifier 110 is reduced to save power, response speed decreases and becomes insufficient for the change in the load condition. On the other hand, when current consumption of the second error amplifier 110 increases, current consumption in the light-load operating mode increases, shortening the life of a battery serving as a power source for the system.